2.4. Establishing Fluid Outflow
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During BESS/UBE, I maintain continuous saline circulation to preserve clear endoscopic visualization. Bleeding from muscle, bone, or epidural vessels rapidly impairs visibility when outflow is insufficient. I therefore monitor both fluid inflow and its free exit through the working portal.
My irrigation system
In my practice, I use gravity-fed irrigation without a pump, connecting two 3 L saline bags simultaneously. In the usual configuration, I position the lower edges of the bags approximately 170 cm above the floor.
This height does not reflect the actual pressure in the working space. Pressure depends on the vertical difference between the fluid level in the bags and the operative field, the resistance of the system, and the patency of the outflow.
When a greater flow is needed, I may briefly raise the bags only if free outflow is maintained, then return them to their usual position.
Free outflow is the essential condition
Fluid must exit freely through the working portal. Outflow is affected by the size of the skin and fascial openings, the condition of the soft tissues, and instrument positioning. Bag height alone or a preset pump pressure does not guarantee safe pressure in the working space.
I do not compensate for impaired outflow by raising the bags. I first stop manipulating and address the cause of the outflow obstruction. I resume work only after outflow has been restored.
If soft tissue swelling increases or a durotomy is suspected, I do not continue forcing fluid into the space: I assess the cause and coordinate further steps with the anesthesiologist.
What I check when outflow is impaired
I first check whether outflow is obstructed by the edges of the skin or fascial opening, soft tissues, or an instrument in the working portal. Together with the anesthesiologist, I assess the adequacy of muscle relaxation.
I usually choose skin incisions 7–10 mm in length. If outflow is insufficient, I adjust the size of the working portal. I always make a cruciate incision in the thoracolumbar fascia—a longitudinal and a transverse incision with depth control. I enlarge the portal tract in a controlled manner, primarily by blunt dilation, avoiding excessive tissue injury.
If needed, I carefully remove tissue that mechanically obstructs the working portal under visual control. I do not remove subcutaneous fat routinely or blindly. After correction, I recheck free outflow and visibility of the operative field.
References
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3. Hong YH, Kim SK, Hwang J, et al. Water Dynamics in Unilateral Biportal Endoscopic Spine Surgery and Its Related Factors: An In Vivo Proportional Regression and Proficiency-Matched Study. World Neurosurg. 2021;149:e836–e843. DOI: 10.1016/j.wneu.2021.01.086.
4. Kang MS, Park HJ, Hwang JH, Kim JE, Choi DJ, Chung HJ. Safety Evaluation of Biportal Endoscopic Lumbar Discectomy: Assessment of Cervical Epidural Pressure During Surgery. Spine (Phila Pa 1976). 2020;45(20):E1349–E1356. DOI: 10.1097/BRS.0000000000003585.
5. Vargas RAA, Hagel V, Zhang X, et al. Durotomy- and Irrigation-Related Serious Adverse Events During Spinal Endoscopy: Illustrative Case Series and International Surgeon Survey. Int J Spine Surg. 2023;17(3):387–398. DOI: 10.14444/8454.
6. Yu D, Jeon I, Kim SW. Real-Time Water Pressure Monitoring in Unilateral Biportal Endoscopic Spine Surgery. Neurospine. 2025;22(3):812–818. DOI: 10.14245/ns.2550648.324.
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